Method and apparatus for disposing dust produced during amorphous silicon film-forming process

ABSTRACT

A method and an apparatus of disposing dust produced during a process for forming an amorphous silicon film, which method and apparatus, for the purpose of safely and quickly collecting the dust and of making it possible to form the collected dust to a solidified mass or pellet form to facilitate subsequent handling thereof, comprises: dust-capturing means for collecting the dust adhering to either the entire or partial portions of the film-forming unit without causing scattering of the dust to the outside of the apparatus; a dust hopper capable of receiving, through a pipeline, the dust collected by the dust-capturing means; a bucket filled with a trifloro-trichloro-ethane liquid to render to a slurry-form the dust supplied thereinto an appropriate rate from the dust hopper; and a heat-distilling means to distill the trifloro-trichloro-ethane liquid by heating the bucket to retain therein solidified mass or pellets of dust.

This is a continuation of copending application Ser. No. 06/897,157filed on Oct. 23, 1986 now abandoned, which was a divisional ofapplication Ser. No. 06/733,948 filed May 14, 1985, now U.S. Pat. No.4,642,208.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a method and an apparatus for disposingdust produced during the process of forming an amorphous silicon (a-Si)film, and more particularly it pertains to a method and an apparatus fordisposing a large amount of dust which is produced during the formationof an a-Si film by relying on photo-CVD (Chemical Vapor Deposition)technique or plasma CVD technique.

(b) Description of the Prior Art

Since the discovery of utility of a-Si which is obtained bydecomposition of silane gas by a glow discharge, there have beenextensively conducted research and development in the matter ofapplications of and putting into practice of a-Si, and as a result amarked progress is noted at present in the improvement of products usinga-Si as a constituent substance. When viewed from the standpoint ofproduction of a-Si, however, there are not a few important problems leftto be solved.

One of them concerns improvement of the deposition rate of a-Si duringits production. This problem, however, has been solved already as statedin, for example, Japanese Patent Preliminary Publication Nos. Sho59-193265 and 59-193266 of the same inventors as for the presentapplication.

The second problem concerns the handling or disposal of the large amountof dust which is produced during the formation of a-Si films by glowdischarge of silane gas. Since, in the past, a-Si itself was still inits way for development, it has been the general idea that the formationof a-Si films has been conducted only on the experimental basis inlaboratories, and thus the manufacture of a-Si on a commercial massproduction basis at a manufacturing plant has been very limited, if any.In the past, the disposal of dust which was produced in each experimentrelied on an unsophisticated manner such that the dust was subjected tosuction by using a small-sized vacuum cleaner or like means and thecollected dust was discarded directly. When it is intended to make amass-production of a-Si photoreceptors, however, the disposal of thedust becomes a very important task.

The reason therefor is due to the special physical property of silicondust. Unless an efficient and effective method for the disposal of thisdust is established, the production of a-Si will not become possible tobe done on an industrial basis. As a preliminary measure for dealingwith the large amount of silicon dust which entails the manufacture ofa-Si, the present inventors have already solved the problem concerninga-Si film formation itself. The most important matter, however, can beboiled down to only one thing which is how the dust thus produced shouldbe disposed. The very specific property of this dust is as follows.

(A) The dust consists of a light-weight, very fine particles of powderform which is unsteady and is ready to fly and scatter aroundimmediately when contacted by a slight current of air, and willcontaminate the environment, and thus it gives rise to a problem in thesanitary management for the workers involved in the manufacture.

(B) a-Si dust is ignited quite easily by a small source of fire such asflame of a match, lighted cigarette, sparkling, or spark from a drivenrivet. The produced dust in itself has no spontaneous ignitability. Onceignited from a near by flame, however, the dust will combust furiouslylike fireworks, and the burning dust will easily scatter and fly around,so that this flame of buring dust will easily induce a fire onsurrounding things. In case a large amount of dust is present, it willinstantaneously catch fire and the flame will spread extensivelythroughout the area like an explosive or a firework, and accordingly thedust is very dangerous.

(C) Once ignited, the burning dust cannot be extinguished according tothe present technique unless the surrounding air is completely shut off.

(D) a-Si dust has no affinity to water and the particles of dust do notdisperse in water, but they remain always in the state of floating onthe surface of water, and thus it is not possible to deal with the dustby using water.

(E) Accordingly, the dust, once ignited, will not become extinguished bymerely sprinking water thereonto, but it will continue to burn in thestate of floating on the surface of water. Also, in the presence ofwater, the dust will rather vigorously burn while developing a scorchingheat. Thus, the burning dust would bring about the same phenomenon as ifby sprinking oil onto fire if water is sprinkled onto the site ofcombustion of a large amount of dust.

(F) Although the dust has no affinity to water, it has affinity toorganic solvents in general, and it disperses in these solvents ofliquid form. However, the dust reacts with lower alcohol and produceshydrocarbon gas, silane gas and hydrogen gas, and such a gas couldignite spontaneously. Thus, the dust is dangerous, and one must avoidmixing the dust with an inflammable organic solvent.

(G) a-Si dust has a smooth flowability like a liquid. Even when dust isapplied with a pressure externally, the dust will only displace around,and it is quite difficult to subject the dust to a compression pressing.

(H) Though depending on a-Si film-forming conditions employed, the dustpresents a brown color in general. When the dust is combusted, the burntdust will still remain as a powder of whitish brown color. This remnantis intensively hydrophilic, and disperses well in water, but will remainas sediments at the bottom of water when left over. The sediments aresilicon oxide, which is of a same quality as soil.

As stated above, a-Si dust in its fine powder form is extremelydangerous, and is very difficult to handle. As such, the dust which isproduced inevitably during mass production in an a-Si manufacturingplant will not lead to the solution of problems merely by collecting thedust which is produced. In the event that the handling or disposal ofdust is not done in an appropriate way, there will arise not onlyproblems related to environmental pollution but also a danger ofdestroying the surrounding things.

As will be appreciated from the foregoing statement, in the past thecollection per se of dust has really been an extremely dangerousoperation, and the prior method of its disposal has been just todirectly combust the dust. Thus, it has not been possible to place thecombustion under control, and therefore a great danger was involved.Furthermore, after combustion of dust, the remnants remain in the formof another dust or powder. Because of such dangers as mentioned above,it has been the usual technique to introduce and mix the collected dustin an amount of water containing, dissolved therein, a higher alcohol ora surface active agent to render the dust hydrophilic, and thereafterthe resulting substances are dispersed in water. In such a case, ittakes a considerable length of time until the burnt dust is dispersed inwater. Moreover, depending on cases, the dust could develop a chemicalreaction and could produce a poisonous gas. What is more, the liquid perse which contains chemical agents is difficult to dispose, and couldrequire a very large used-liquid treating equipment which, in turn,requires a large running cost.

In case it is intended to manufacture, for example, amorphous siliconphotosensitive devices, it has been the prior technique of such a lowlevel that silane gas is supplied into the reaction chamber, whileapplying radio frequency (RF) to the interior of the reaction chamber todevelop a glow discharge, thereby effecting deposition of a-Si substanceonto the surface of a substrate which is housed therein, and that,during this reaction, the dust which is produced within the reactionchamber is collected merely by a suction means such as a rotary pump(RP), and the collected dust is discarded directly. In the past, therehave been attempted various counter-measures for the danger of silanegas. With respect to the dust of silicon among others, however, thedevelopment of its counter-measure has been rather neglected so far.

SUMMARY OF THE INVENTION

In view of the foregoing state of art, it is the primary object of thepresent invention to provide a method and an apparatus of disposing dustwhich is produced during the process of forming a-Si films, which methodand apparatus being such that, in order to safely and quickly collectsuch dust to facilitate its subsequent handling, the dust can be formedinto a solid mass or pellets.

This object is attained according to the present invention by capturingthe dust without causing the dust to scatter to the outside of theamorphous silicon film-forming system, and then mixing the collecteddust in a heat-distillable non-inflammable liquid having affinity tosilicon to thereby render the captured dust to a slurry state, andthereafter heat-distilling the slurry-form dust mixture to recover onlythe liquid from the mixture substantially completely, and thencollecting the resulting solidfied mass or pellets of dust for recyclingpurpose.

According to a preferred formation of the present invention, the dustdisposing apparatus comprises a vacuum suction means coupled to afilm-forming section for discharging the dust produced within thefilm-forming section to the outside thereof; a dust capturing section tosaid vacuum suction means; a dust hopper connected to the dust capturingsection for receiving the captured dust therefrom; mixing meansremovably coupled to the dumping means and filled with aheat-distillable liquid for rendering the dust flowing thereinto fromthe dumping means to a slurry state; and heat-distilling meansconnectable to said mixing means for heating the mixing means to distillsaid liquid so as to leave, within the mixing means, the resultingsolidified mass or pellets of dust.

According to the present invention, there preferably can be usedtrifloro-trichloro-ethane (C₂ Cl₃ F₃) liquid to serve as theabove-mentioned heat-distillable liquid.

This and other objects of the present invention will become moreapparent during the course of the following detailed description andappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic explanatory illustration showing an embodimentof the a-Si film-forming apparatus provided with a dust capturing meansaccording to the present invention.

FIG. 2 is a diagrammatic explanatory illustration showing the state thata dust capturing means different from that shown in FIG. 1 is attachedto the apparatus and that the interior of the apparatus is beingcleaned.

FIG. 3 is a diagrammatic illustration showing the state that the dustdeposited on the interior-constituting members of the a-Si film-formingapparatus is being cleaned.

FIG. 4 is a diagrammatic explanatory illustration showing the method ofdisposing the captured dust.

FIG. 5 is a diagrammatic explanatory illustration showing an example ofmeans for recovering the c₂ Cl₃ F₃ liquid for rendering the dust tosolidified mass or pellets.

FIG. 6 is a diagrammatic explanatory illustration showing an example ofthe state that pellets of dust are contained in a container.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will hereunder be made with respect to anembodiment to manufacture a photosensitive drum using amorphous silicon.

In FIG. 1, reference numeral 1 represents a substrate drum for formingthereon an a-Si film. In order to realize a film of even thickness, thissubstrate drum 1 is mounted on a mandrel 3 coupled to a motor 7. Withinthis mandrel 3, there may be additionally provided a heater foradjusting the temperature within the substrate. Numeral 2 represents acylindrical metal net-form electrode which is connected to a radiofrequency power supply 5 to develop a glow discharge across theelectrode and the substrate drum 1. Numeral 4 represents a vacuumreaction chamber provided with castors on the underside thereof, and isarranged to be detachable from a base plate 8. Numeral 6 represents asupply source of a starting material such as silane gas. A doping gassuch as H₂ and N₂ is introduced into the reaction chamber 4 during aglow discharge, through a gas ejection tube 11 having a number of gasejecting fine perforations formed through its circumference and beingconnected to the gas supply 6. This gas is supplied while being adjustedof its flow rate via a valve not shown which, in turn, is provided onthe gas supply 6 which may be a container filled with pressurized suchgas. The glow discharge is developed by an application of a radiofrequency power which is supplied across the substrate drum 1 and theelectrode 2 from the radio frequency power supply 5. Numeral 9represents an evacuating means for producing a substantial vacuum, andis constructed to keep a constant operating pressure within the reactionchamber 4, and it is connected to this reaction chamber 4 via a dustcollecting vessel 10 having a filtering function.

In the film-forming apparatus having the above-mentioned structure, thestarting material gas such as silane is decomposed by the application ofa glow discharge thereto, and part of the decomposed gas deposits as ana-Si film on the surface of the drum 1. Also, the solidified substancein the gas atmosphere deposits to the entire surfaces of the inner wallsof the reaction chamber 4 and also around the gas ejection tube 11provided within the reaction chamber, and the remainder passes throughthe drain tube S to be captured into a dust filtering and collectingvessel 10. After completion of the film-forming operation, thesedeposits and the captured dust are cleaned to prepare for the nextfilm-forming operation.

FIG. 2 shows the state in which the reaction chamber 4 is detached fromthe base plate 8 and is connected to another cleaning means 12 having acleaning function. A stable inert gas such as N₂ is introduced, underpressure, into the reaction chamber 4 through a rotary reciprocablenozzle 13 which is connected to a motor M which, in turn, is provided onthe cleaning means 12, to blow away the dust adhering to the surfaces ofthe inner walls of the reaction chamber so as to be discharged to theoutside of the reaction chamber via a suction tube S'.

FIG. 3 shows an example wherein a film-forming unit comprising a mandrel3, a drum 1, a metal net-form electrode 2 and a gas-ejecting tube 11 isexposed into a tightly closed chamber C from a reaction chamber 4 byattaching a base plate 8 to this closed chamber C, and the dust adheringto the electrode 2 and the gas-ejecting tube 11 is perfectly removedwhile sliding respective suction nozzles 14 sideways along the net.Arrangement is provided so that a stable inert gas current such as N₂ isintroduced into this closed chamber C from an upper portion thereof totravel toward its bottom via a gas-discharging pipe P so that thisintroduced gas is discharged through the bottom of this chamber into acapturing vessel via a tube.

It will be needless to say that the respective functions shown in FIGS.2 and 3 may be incorporated directly into the reaction chamber 4 shownin FIG. 1.

In FIG. 4, the dust a which has been captured in the filtering andcollecting vessel 10 of FIG. 1, or the dust captured in either FIG. 2 orFIG. 3 is therefrom invariably collected into a dust hopper 15 via adust flow passageway D. Numeral 16 denotes a bucket which is detachablyconnected to a trifloro-trichloro-ethane liquid supply 18 via anappropriate means 19 such as a dumper, and has been preliminarily filledwith a liquid C₂ Cl₃ F₃. Arrangement is provided so that the neck andmouth portion of this bucket 16 communicates the bottom of the dusthopper 15 so that the dust collected within the hopper will be allowedto drop and flow into this bucket 16 by its own weight. Numeral 17represents a stirring means which can be driven from outside the systemby an appropriate driving power, and is assigned to mix the dust withthe C₂ Cl₃ F₃ liquid in the bucket 16 to render them to a slurry-form ofdust a'.

FIG. 5 shows C₂ Cl₃ F₃ liquid recovering means. The bucket 16 whichcontains the slurry-form of dust which has been mixed with the liquid isdetached from the hopper 15, and then it is heated to an appropriatetemperature of 80° C. or lower by an appropriate heating means 20 tocause evaporation of only C₂ Cl₃ F₃, and the vapor of this substance isthen cooled by a cooling means 22 via a distilling tube 21 so as to bereceived into a C₂ Cl₃ F₃ recovering vessel 18' as a liquid C₂ Cl₃ F₃.The above-mentioned slurry-form dust a' will, when C₂ Cl₃ F₃ completelyevaporates as it is heated, become solidified pellets a" as shown inFIG. 6. These pellets are put into an appropriate container such as abag 23 and are burnt. The dust thus burnt differs in no way fromordinary soil, and accordingly it can be discarded with no difficulty.Also, this burnt dust is a hydrogenized amorphous silicon containing alarge amount of hydrogen. Therefore, the solidified pellets of dustmentioned above may be subjected further to a shaping or pressing stepso that the resulting mass not only may be preserved as it is, but alsoit may be utilized again as a hydrogenized silicon semiconductormaterial. Especially, as stated of the present invention, the burnt dustcan be solidified in the state of not containing any impurity since itscontact with external atmosphere has been avoided. Thus, theabove-mentioned silicon containing a large amount of hydrogen may besubjected to decomposition of water which it contains to thereby develophydrogen therefrom. The hydrogenized silicon in its powder form isdifficult to utilize in an effective way. When solidified hydrogenizedsilicon is applied with light rays or with electric stimulation in waterto activate it, the resulting activated solid matter will develop H₂ gasvery efficiently. Thus, the burnt dust is quite useful also from theaspect of utilizing hydrogen energy.

The foregoing statement may be summarized as follows.

(1) The dust produced during the a-Si film-forming process is collectedin a dust collecting vessel 16 without being exposed to the outside ofthe system. Accordingly, there does not arise any scattering of dust tothe outside of the system, so that there occurs no problem of dustpollution nor a problem concerning sanitation for the workers involved.Also, dust is carried through a safety pipe line D and has no contactwith the outside of the system, so that there is no risk of beingignited and combusted. Furthermore, the blowing of gas into the reactionchamber 4 is performed by using an inert gas such as nitrogen gas, sothat there is eliminated any fear for combustion of dust, and a greatersafety is insured. Moreover, the electric system which is associatedwith the manufacturing system is of an explosion-free structure for thesake of safety.

(2) Dust is collected in the dust hopper 15 by relying on the powdertransportation technique. The interior of the whole system ispreliminarily filled with an inert gas such as nitrogen so that therespective members are surrounded by the inert gas. When a certainamount of dust a has accumulated in the dust hopper 15, the dumper 19 isopened to allow the dust a to drop and flow into the C₂ Cl₃ F₃-containing vessel 16. Thus, the dangerous dust will not scatter eitherentirely or partially to the outside of the system, and accordinglysafety is insured.

(3) Dust has a good affinity to C₂ Cl₃ F₃ liquid, and will becomeslurry-form when mixed therewith. The C₂ Cl₃ F₃ liquid never reacts withdust a. Also, the C₂ Cl₃ F₃ liquid itself is not inflammable, and willnot combust spontaneously. The said slurry-form dust is hard to getignited. Once ignited, however, the mass of dust slurry will combust andejects poisonous halogen gas, and so caution must be taken to the heatof fire which may be present in the vicinity of the slurry of dust.Being slurry-form, however, there will not occur scattering of dust asin the case of fine powder form of dust.

(4) It is possible to recover only the C₂ Cl₃ F₃ liquid by using a C₂Cl₃ F₃ recovering means. The recovered C₂ Cl₃ F₃ liquid can be usedagain in the disposal of dust in the subsequent operation, and thereforerepletion of the liquid only in an amount which has been lost is enough,and thus a big economic effect is achieved. The C₂ Cl₃ F₃ per se is notdischarged from the system, so that no problem of environmentalpollution will arise.

(5) After C₂ Cl₃ F₃ has evaporated completely within the system, thereis formed pellets a" of solidified silicon. Since they are in the formof clods, there is no fear for their scattering around, and accordinglythey are easy to handle. Even these pellets of silicon can burn ifignited. However, they will not develop an explosively severe combustionlike the untreated dust, but instead, they will burn only gently. Also,as stated above, the pellets will not scatter around as does the dust offine powder form, and therefore there is no fear for induced ignition,and thus pellets are much safer as compared to silicon in the state offine powder. Furthermore, after combustion, the pellets will present astate like cinders of burnt coal, and they are easy to discard.

(6) In case the dust has been solidified in such a manner as thatdescribed above, the solidified dust has reduced its volume to 1/10-1/20of its original volume of powder, which is quite advantageous andconvenient for a volume for disposal.

Description has been made of an embodiment with respect to the disposalof dust which is produced during the manufacture of a photosensitivedrum using amorphous silicon. It should be noted, however, that thepresent invention is not limited thereto. More particularly, the presentinvention is applicable equally effectively to the disposal of dustwhich is produced as a by-product when an a-Si film or a polycrystallinesilicon film is formed by decomposing a gaseous silicon compoundcontaining silane by a glow discharge relying on, for example, plasmaCVD technique to manufacture a solar battery or other active devices.

What is claimed is:
 1. A system for disposing of dust produced during a process of forming an amorphous silicon film by decomposing a starting material gas containing silane gas as its principal component, comprising:a unit for collecting the dust including:vacuum suction means coupled to a film-forming section to discharge the dust produced within said film-forming section to the outside of this section; a dust-capturing section coupled to said vacuum suction means; a dust hopper connected to said dust-capturing section to receive the captured dust; and dumping means, provided at a bottom portion of said dust hopper, for dumping dust received in the dust hopper, said dumping means being adapted to receive a detachable mixing means; and heat-distilling means, adapted to receive a detachable mixing means, for heating said mixing means; and mixing means, adapted for detachably coupling to said dumping means and said heat-distilling means and including a heat-distillable liquid therein, for receiving dust from said dumping means when coupled to said dumping means and mixing said dust with said liquid to transform said dust to a slurry-form and for heating and distilling said liquid when coupled to said heating means to convert the slurry-form dust therein into a final disposal product comprising a solidified mass or pellets of dust.
 2. An apparatus according to claim 1, in which:said liquid is a trifloro-trichloro-ethane liquid.
 3. An apparatus according to claim 1, in which:said dust hopper is filled with an inert gas.
 4. A system for disposing of dust produced during a process of forming within a reaction chamber an amorphous silicon film by decomposing a starting material gas containing silane gas as its principal component, comprising:a unit for collecting the dust including:cleaning means, detachably coupled to the reaction chamber, for discharging dust adhering to inner surfaces of walls of said reaction chamber to the outside of the reaction chamber; a dust hopper connected to said cleaning means to receive the dust discharged from said reaction chamber; and dumping means provided at a bottom portion of said dust hopper for dumping dust received in the dust hopper, said dumping means being adapted for coupling a detachable mixing means thereto; heat-distilling means, adapted to receive a detachable mixing means containing a heat-distillable liquid, for heating the mixing means to distill said liquid; and mixing means, adapted for detachably coupling to said dumping means and said heat-distilling means and including a heat-distillable liquid therein, for mixing dust received from said dumping means to transform said dust to a slurry-form, when coupled to said dumping means with said liquid, and for distilling said liquid and converting the slurry-form dust therein into a final disposal product comprising a solidified mass or pellets of dust, when coupled to said heat-distilling means.
 5. An apparatus according to claim 4, in which:said cleaning means comprises a motor, and a nozzle coupled to said motor and capable of making rotating and reciprocating movements within said reaction chamber and assigned to introduce an inert gas into the reaction chamber therethrough.
 6. An apparatus according to claim 4, in which:said liquid is a trifloro-trichloro-ethane liquid.
 7. An apparatus according to claim 4, in which:said dust hopper is filled with an inert gas.
 8. A system for disposing of dust produced during a process of forming an amorphous silicon film by decomposing a starting material gas containing silane gas as its principal component, comprising:a unit for collecting the dust including:suction nozzle means for sucking the dust, said suction nozzle means being movably provided with a tightly closed chamber to discharge dust adhering to a film-forming unit housed in said closed chamber to the outside of this latter chamber; a dust hopper connected to said suction nozzle means to receive the dust discharged from said closed chamber; and dumping means provided at a bottom portion of said dust hopper for dumping the received dust, said dumping being adapted for coupling a detachable mixing means thereto; heat-distilling means, adapted to receive a detachable mixing means having a heat-distillable liquid therein, for distilling said liquid; and mixing means, adapted for detachably coupling to said dumping means and said heat-distilling means and including a heat-distillable liquid therein, for receiving dust from said dumping means when coupled to said dumping means and mixing said dust with said liquid to transform said dust to a slurry-form and for transforming the slurry-form dust therein into a final disposal product comprising a solidified mass or pellets of dust when coupled to said heat-distilling means.
 9. An apparatus according to claim 8, in which:a current of an inert gas is formed in said closed chamber to travel from its upper portion toward its bottom portion.
 10. An apparatus according to claim 8, in which:said liquid is trifloro-trichloro-ethane liquid.
 11. An apparatus according to claim 8, in which:said dust hopper is filled with an inert gas. 